This invention relates to sound absorbing liners and particularly to the deep backing cavity liner configurations for jet engine applications.
As is well know in the acoustic art, the liner must be tuned to the frequency of the noise which is intended to be attenuated. In a turbofan engine, a dominant noise in the primary flow is that produced by the combustor which falls within a relatively low frequency range, say below 1000 Hz. It is well known that in a deep back cavity liner the depth of the cavity is directly related to the frequency in that as the cavity depth is increased the frequency of maximum noise attenuation decreases.
The problem, thus, is obvious where there is a space limitation, the depth available may not accommodate a deep enough cavity and thus prohibit the atenuation at the necessary low frequency range. Thus, a 6 inch depth backing cavity is not possible if less than 6 inch are available. Another well known technique for decreasing resonant frequencies is used in a simple Helmholtz reasonator. The neck depth is increased which increases the effective mass reactance which decreases the natural frequency of the resonator. A similar technique can be applied to the engine acoustic treatment which consits of increasing the thickness of the facing to sheet. This again has its limitations since additional weight of the facing sheet is a severe penalty on engine performance, which may be intolerable, in addition to the space limitation noted above.
I have found that I can obviate the problems noted above by providing on the facing sheet a depending lip at the entrance slot extending into the cavity for providing space defining means for supporting a column of air. This increases the mass reactance, lowers the tuning frequency and permits the design to give substantially the same sound attenuation that would otherwise be obtained attendant an increase of the depth of the cavity. Furthermore, by making the acoustic treatment a slotted construction, the smallest opening on the surface of the acoustic treatment in the direction of air flow is provided. Aerodynamic losses due to back flow and turbulence increase as the opening increases. Therefore, the proposed construction provides the minimum aerodynamic loss for a required acoustic treatment design.
In another embodiment the column shape is modified so as to increase the mass reactance without increasing the depth of the column.
This invention also contemplates controlling the dimensions of the entrance slot and the depth of the entrance column to maximize acoustic treatment in a nonuniform element where the cavity depth is inherently limited as would be the case of a plug of the tailpipe of an engine.